(i) Field of Invention
This invention relates to removal of organically-bound chlorine from bleach plant effluents using alkaline hydrolysis.
(ii) Description of the Prior Art
Presence of chlorinated organic compounds in bleached kraft mill effluents has been recognized as an important environmental issue. Due to a low rate of mineralization, the high molecular weight chlorolignins are known to persist in an aquatic environment for prolonged periods of time. Regulations to decrease the discharge of adsorbable organic chlorine (AOX) in Canada and abroad have been introduced.
Several approaches are practiced for AOX control at bleached kraft mills. The in plant control is based on reducing the AOX formation during the bleaching by using extended delignification in the digester, oxygen delignification, high ClO.sub.2 substitution, and improved pulp washing before bleaching. External effluent treatment can also reduce AOX substantially. Removal of AOX up to 50% has been reported for both the aerated lagoon and activated sludge treatments. The removal of AOX in aerated lagoons is believed to be accomplished principally by physical-chemical means, namely coagulation and absorption on biomass followed by removal of the biosolids by secondary clarification or by settling into the benthal zone. In the latter case, anaerobic mineralization of these compounds then apparently proceeds in the upper layer of the sediment close to the sediment-water interface.
Another approach to AOX mineralization is by hydrolysis of organic chlorine under alkaline conditions. The aliphatically bound chlorine is usually more susceptible to such hydrolysis than that bound aromatically, but the latter species can also be hydrolyzed under certain conditions, as shown by Migita et al, J. Japan Wood Res. Soc. L, 55 (1955) and Braddon and Dence, TAPPI 51 (6) 249 (1968). The degree of chlorine hydrolysis depends strongly on: the location of the chlorine atoms on the chlorolignin molecule; and the strength of the alkali solution used. Thus the chlorine in .alpha. position in dichloroisoeugenol was found to be removed completely upon one hour hydrolysis at 60.degree. C. in water alone. The chlorine removal from 3,4,5,6-tetrachlorocatechol and 4,5-dichlorocatechol was also relatively fast, typically 52-72% removal after 3 hours at about 60.degree. C., when the treatment was performed in 0.5M sodium hydroxide. This can be explained largely by the susceptibility of the catechol nucleus to oxidation by traces of oxygen to chloro-o-quinones. It has been suggested that the hydroxyquinones formed might serve, in the absence of oxygen, as oxidizing agents for the original chlorocatechols and this might help to dechlorinate these compound completely upon prolonged alkaline treatment. Similarly, the 3-chloro-5-methyl-o-quinone and 4-chloro-5-methyl-o-quinone were reported to undergo a chlorine loss, between 30 and 75%, respectively, when hydrolysed in 0.5M NaOH at 60.degree. C. for one hour. The aliphatic oxidation products of chloro-o-benzoquinone, such as the chloromuconic acid derivatives, also lose chlorine upon NaOH hydrolysis, particularly that substituted in .alpha. position. On the other hand, the chlorine hydrolysis of .beta.-chloromuconic acid proceeds relatively slowly while the hydrolysis of .beta.-chloroacrylic acid was found to be practically insignificant. The chlorinated compounds most resistant to alkaline hydrolysis are those containing guaiacyl and veratryl nuclei. Compared to catechols, these structures are less susceptible to oxidation to quinones and thus the oxidative hydrolysis, typical for catechols, can take place only after replacement of methoxyl groups by hydroxyl groups.
There are several reactions that are used in organic chemistry for elimination of chlorine from chlorinated organic compounds. During the nucleophilic hydrolysis of alkyl and aryl chlorides, as well as acid chlorides, the organic chlorine is expelled as chloride and its place is taken by another basic group, such as hydroxyl. Saturated alcohols or organic acids are thus produced.
The dehydrochlorination represents removal of both chlorine and hydrogen from chlorinated organic molecules. Typically, the breaking of C--H and C--Cl occurs in unsymmetrical fashion in the sense that hydrogen relinquishes both electrons to carbon while chlorine retains both electrons. The electrons left behind by hydrogen are available to form the second bond (.pi. bond) between the carbon atoms and this results in formation of alkene structure. The energy required for breaking the C-H bond is supplied from (i) formation of a bond between the hydrogen ion and the strongly basic hydroxide ion, (ii) formation of the bond between the two carbon atoms, and (iii) energy of solvation of the chloride ion. Dehydrochlorination proceeds best in alcoholic KOH. The function of this medium is to form the strongly basic alcoholate C.sub.2 H.sub.5 OK and to solubilize the starting organic compound. In cases when the organic compounds are water soluble, such as chlorinated lignin, the reaction is expected to proceed in an aqueous medium.
It is also known in organic chemistry that the first-order alkaline hydrolysis of .beta.,.beta.'-dichlorodiethyl sulphide (DCDES) is much faster than that of other alkyl halides. This situation is typical of substitution nucleophilic unimolecular S.sub.N 1 reaction, which proceeds usually in two steps, via organic carbonium cation. In the case of DCDES, the formation of carbonium cation is improbable because it would have to be formed on the primary carbon and this type of cation is highly unstable. Another possibility for elucidating the fast hydrolysis of DCDES is formation of a highly reactive cyclic sulphonium ion. The strongly nucleophilic sulphur is believed to help displace chlorine from the molecule of DCDES.
Attempts to reduce the content of organic chlorine by alkaline treatment have been already made by at least three research groups. Holmberg et al, Svensk Papperstd. 86 (15), R152 (1983) treated E.sub.1 ultra filtration (UF) concentrates at temperatures up to 100.degree. C. and pH up to 12.5. About 50% of the organic chlorine was removed after 2 hours treatment. An oxygenation process (150.degree. C., 1.0 MPa, 40-60 minutes) was applied to UF concentrates (E.sub.1 ) by Sun et al TAPPI 72 (9), 209 (1989). The reactor was pressurized with oxygen and mixing at 450-900 rpm to increase the oxygen transfer. Under these conditions, the TOCl removal was 70-80% and colour was removed by 60-70%. Bottger et al "Dechlorination and Biological Treatment of Chlorinated Organic Substances", 4th Int. Symp.--Wood and Pulping Chemistry Paris, France, 1989, studied pretreatment of sulphite chlorination effluent at a pH of 11 and a temperature of 60.degree. C. After 1 hour treatment, the AOX reduction was 50% and this was increased to about 70% following aerobic treatment.